Organic light emitting display device and method of fabricating the same

ABSTRACT

An apparatus for providing a driving signal to an organic light emitting diode in an image display device includes gate lines for transferring previous and current gate signals, respectively, in a sequential process for providing the driving signal to the organic light emitting diode, a data line for transferring a data signal for displaying images on the image display device, a first switching transistor including a conduction path for transferring the data signal from the data line in response to the current gate signal; a second switching transistor including a conduction path for transferring a reference signal externally supplied in response to the previous gate signal, a third switching transistor including a conduction path for transferring the data signal provided from the first switching transistor in response to a state of the second switching transistor, and a fourth switching transistor including a conduction path for receiving a bias voltage and generating the driving signal to the organic light emitting diode in response to one of the reference signal from the second switching transistor and the data signal from the third switching transistor. The third and fourth switching transistors have switching characteristics substantially identical to each other.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an organic light emitting displaydevice, and more particularly, to a device for driving pixels of anorganic light emitting display device and a method of fabricating thesame.

2. Description of the Related Art

Cathode ray tube (CRT) devices have been widely used for various kindsof image display devices. Recently, liquid crystal display (LCD) deviceshave emerged as an alternative display means for, especially, portableequipments, computer monitors, etc. However, CRT devices are generallyheavy and have a big size, and LCD devices also have some unsatisfactoryfactors such as mediocre brightness, low efficiency, etc. In addition,LCD devices have such a drawback that images may have poor views at theside of an LCD device.

Thus, there have been made various developments for an image displaydevice, as a new generation display means, having a lighter weight, aslimmer size, an affordable price, better efficiency, etc. One of suchnew generation display devices is an organic light emitting display(OLED) device. The OLED devices utilize the electroluminescencecharacteristics of certain organic compounds or high polymers, whichemit light in response to electric current applied thereto. In the OLEDdevices, no backlight device is necessary for providing light to adisplay panel, which is required for the LCD devices. Thus, the OLEDdevices advantageously have a lighter weight, a smaller (and slimmer)size, a lower cost, etc. and are more readily fabricated compared withthe LCD devices. In addition, the OLED devices may have superiorbrightness and a larger viewing angle.

FIG. 1 is a circuit diagram illustrating a conventional driving circuitfor an OLED device, and FIG. 2 is a graphical view of signal waveformsapplied to the driving circuit in FIG. 1. Referring to FIGS. 1 and 2,the conventional driving circuit for an OLED device includes a switchingtransistor Q_(S) having a gate and a source connected to a gate line Gqand a data line Dp, respectively, a storage capacitor Cst having oneterminal connected to a drain of the switching transistor Q_(S) and theother terminal connected to a bias voltage Vdd, and a driving transistorQ_(D) having a gate connected to the drain of the switching transistorQ_(S) and a source connected to the bias voltage Vdd.

A driving signal is provided from a drain of the driving transistorQ_(D) to an organic light emitting diode OLED. The organic lightemitting diode OLED has one end connected to a drain of the drivingtransistor Q_(D) and the other end connected to a common electrodevoltage V_(COM). Generally, the switching transistor Q_(S) is an N-typethin film transistor that is turned on by applying a high-level voltagesignal to its gate, and the driving transistor Q_(D) is a P-type thinfilm transistor that is turned-off when the high-level voltage signal isapplied to its gate.

In the operation the driving circuit in FIG. 1, when the switchingtransistor Q_(S) is turned on by a gate signal provided through the gateline Gq, a data signal from the data line Dp is transferred through theconduction path of the switching transistor Q_(S) to the gate of thedriving transistor Q_(D) as a gate voltage. The gate voltage ismaintained for one frame due to the storage capacitor Cst. At this time,channel conductance of the driving transistor Q_(D) is determined by thegate voltage applied to the gate and the bias voltage applied to thesource of the driving transistor Q_(D). Also, the intensity of a voltageapplied between the ends of the organic light emitting diode OLED isdetermined based on a voltage distribution of the organic light emittingdiode OLED with respect to the voltage between the bias voltage Vdd andthe common electrode voltage V_(COM), where the organic light emittingdiode OLED and the driving transistor Q_(D) are connected each other inseries. The organic light emitting diode OLED emits light in response tocurrent flowing therein, which is corresponding to the intensity of thevoltage determined based on the voltage distribution.

Thus, even if the same data signal is applied to the gate of the drivingtransistor Q_(D) so that the gate-source voltage V_(GS) of the drivingtransistor Q_(D) has an identically value in different driving circuitsfor different pixels of the OLED device, the voltage distribution mayvary depending on characteristics of the driving transistor Q_(D) indifferent pixels so that the intensity of the voltage between the endsof organic light emitting diode may vary as well. As a result, thecurrent flowing the organic light emitting diode OLED may be differentin different pixels of the OLED device. Such variation in the currentflowing the organic light emitting diode OLED may cause deterioration inbrightness of pixels and display quality of the OLED device.

Therefore, it is desired that the driving circuit of an OLED device isimproved so that every organic light emitting diode in the respectivepixels of the OLED device receives the same driving current in responseto the same data signal so as to emit the same amount of light.

BRIEF SUMMARY OF THE INVENTION

The present invention provides a pixel driving unit for an OLED devicethat compensates for characteristics of a driving thin film transistorin the pixel driving unit to improve the display quality of the OLEDdevice. The present invention also provides a method of fabricating sucha pixel driving unit for an OLED device.

In one aspect of the invention, a pixel driving unit for providing adriving signal to an organic light emitting diode in an OLED deviceincludes first and second control lines for transferring previous andcurrent control signals, respectively, in a sequential process forproviding the driving signal to the organic light emitting diode, a dataline for transferring a data signal for displaying images on the imagedisplay device, a first switching device including a conduction path fortransferring the data signal from the data line, the conduction path ofthe first switching device being controlled by the current controlsignal from the second control line, a second switching device includinga conduction path for transferring a reference signal externallysupplied, the conduction path of the second switching device beingcontrolled by the previous control signal from the first control line, athird switching device including a conduction path for transferring thedata signal provided from the first switching device, the conductionpath of the third switching device being controlled by a state of thesecond switching device, and a fourth switching device including aconduction path for receiving a bias voltage and generating the drivingsignal to the organic light emitting diode, the conduction path of thefourth switching device being controlled by one of the reference signalfrom the second switching device and the data signal from the thirdswitching device.

In the pixel driving unit, the third and fourth switching devices mayhave switching characteristics substantially identical to each other.Also, a capacitor may be included for being charged with the biasvoltage and for providing a voltage signal to control the conductionpath of the third switching device. The first, second, third and fourthswitching devices may be first, second, third and fourth thin filmtransistors, respectively, each having a conduction path between asource and a drain and a gate for receiving a control signal to controlthe conduction path. The first and second control signal lines may befirst and second gate lines, respectively, and the previous and currentcontrol signals may be previous and current gate signals, respectively.

In another aspect of the present invention, an organic light emittingdisplay device includes gate lines to which an active gate line issequentially supplied, data lines to which data signals are applied todisplay images on the organic light emitting display device and pixeldriving units each of which provides a driving signal to a correspondingOLE diode in association with a pair of the gate lines and a pair of thedata lines, in which each of the pixel driving units has a drivingtransistor having a conduction path with one terminal receiving a biasvoltage and the other terminal providing the driving signal to thediode, a first switching transistor having a conduction path fortransferring a reference signal, the conduction path of the firstswitching transistor being controlled by a previous gate signal, and asecond switching transistor having a conduction path for transferring adata signal, the conduction path of the second switching transistorbeing controlled by a state of the first switching transistor. Theconduction path of the driving transistor may be controlled by one ofthe reference signal from the first switching transistor and the datasignal from second switching transistor. The gate lines may include adummy gate line for providing a gate signal to the first switchingtransistor of a first one of the pixel driving units.

In another aspect of the present invention, there is provided a methodfor fabricating a semiconductor device for providing a pixel drivingsignal in an organic light emitting display device. The method includesproviding an insulation substrate, forming on the insulation substrate afirst amorphous silicon thin film transistor for providing the pixeldriving signal to an organic light emitting diode, forming on theinsulation substrate a second amorphous silicon thin film transistor fortransferring a data signal to control a switching function of the firstamorphous silicon thin film transistor, crystallizing the first andsecond amorphous silicon thin film transistors by performing a laserscan on the first and second amorphous silicon thin film transistor, andtransforming the first and second amorphous silicon thin film transistorinto first and second polysilicon thin film transistors, respectively,by consummating the crystallizing step. The first and second polysiliconthin film transistors may have characteristics substantially identicalto each other.

These and other objects, features and advantages of the presentinvention will become apparent from the following detailed descriptionof the exemplary embodiments thereof, which is to be read in conjunctionwith the accompanying drawings, wherein like elements are designated byidentical reference numbers throughout the several views.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other advantages of the present invention will becomereadily apparent by reference to the following detailed description whenconsidered in conjunction with the accompanying drawings wherein:

FIG. 1 is a circuit diagram illustrating a conventional driving circuitfor an OLED device;

FIG. 2 is a graphical view of signal waveforms applied to the drivingcircuit in FIG. 1;

FIG. 3 is a circuit diagram illustrating a pixel driving unit for anOLED device according to a first embodiment of the present invention;

FIG. 4 is a graphical view of signal waveforms applied to the pixeldriving unit in FIG. 3;

FIGS. 5A and 5B are schematic diagrams for describing the operation ofthe pixel driving unit in FIG. 3;

FIG. 6 is a circuit diagram illustrating a pixel driving unit for anOLED device according to a second embodiment of the present invention;

FIG. 7 is a circuit diagram illustrating a pixel driving unit for anOLED device according to a third embodiment of the present invention;

FIG. 8 is a circuit diagram illustrating multiple pixel driving units ofan OLED device arranged in association with gate and data linesaccording to an embodiment of the present invention;

FIG. 9 is a circuit diagram illustrating a pixel driving unit for anOLED device according to a fourth embodiment of the present invention;

FIG. 10 is a circuit diagram illustrating a pixel driving unit for anOLED device according to a fifth embodiment of the present invention;

FIG. 11 is a circuit diagram illustrating a pixel driving unit for anOLED device according to a sixth embodiment of the present invention;

FIG. 12 is a graphical view of signal waveforms applied to the pixeldriving unit in FIG. 11;

FIGS. 13A and 13B are schematic diagrams each illustrating two thin filmtransistors fabricated according to an exemplary embodiment of thepresent invention;

FIG. 14 is a plan view of the pixel driving unit in FIG. 3; and

FIGS. 15 and 16 are cross-sectional views of the pixel driving unittaken along lines A-A′ and B-B′, respectively, in FIG. 14.

DETAILED DESCRIPTION OF THE INVENTION

Detailed illustrative embodiments of the present invention are disclosedherein. However, specific structural and functional details disclosedherein are merely representative for purposes of describing exemplaryembodiments of the present invention.

FIG. 3 is a diagram illustrating a pixel driving unit of an organiclight emitting display (OLED) device according to a first exemplaryembodiment of the present invention, and FIG. 4 is a graphical view ofsignal waveforms applied to the pixel driving unit in FIG. 3.

In FIG. 3, the pixel driving unit of an OLED device includes fivetransistors, for example, first to third thin film transistors T₁-T₃each having a switching function and fourth and fifth thin filmtransistors T₄, T₅ each having a driving function. The pixel drivingunit also includes a storage capacitor Cst for storing electric chargeand an OLED diode for emitting light in response to a driving signalapplied from the fifth thin film transistor T₅. The OLED diode has afirst terminal receiving the driving signal from the fifth transistor T₅and a second terminal connected to a common electrode voltage V_(COM).In this embodiment, the five thin film transistors T₁-T₅, the storagecapacitor Cst, and the OLED diode are the elements mainly constitutingthe pixel driving unit for a unit pixel of an OLED device.

The pixel driving units are arranged in association with gate lines anddata lines of the OLED device such that each pixel driving unit isdisposed in a region surrounded by adjacent gate lines and adjacent datalines. Power supply lines each providing a bias voltage Vdd for the OLEDdevice is also arranged such that each power supply line is parallelwith a corresponding data line D_(p+1). A selected number of the pixeldriving units of the OLED device are connected to the power supply lineVdd, and the number of the pixel driving units is equal to the number ofthe gate lines. The power supply line Vdd may be formed as a singlemetal layer (e.g., MoW layer) or dual metal layers (e.g., MoW layer andAlNd layer).

As shown in FIG. 3, one pixel driving unit (or one unit pixel) isdefined by p_(th) and (p+1)_(th) data lines and (q−1)_(th) and q_(th)gate lines in an OLED device having a resolution of m×n×3. In the pixeldriving unit, for example, the first and second thin film transistorsT₁, T₂ are N-type transistors, which are each turned on when a gatesignal higher than the threshold voltage of a corresponding transistoris applied to its gate. The third to fifth thin film transistors T₃-T₅are P-type transistors, which are each turned on when a gate signallower than the threshold voltage of a corresponding transistor isapplied to its gate. In particular, the first thin film transistor T₁has a gate connected to a present gate line G_(q) and a source connectedto a data line Dp. The first thin film transistor T₁ transmits a datasignal, which is inputted through the source, to the third thin filmtransistor T₃ through the drain thereof in response to the gate signalapplied through the present gate line G_(q).

The second thin film transistor T₂ has a gate connected to a previousgate line Gq⁻¹ and a source connected to a reference voltage line towhich a reference voltage V_(ref) is supplied. The second thin filmtransistor T₂ transmits the reference voltage V_(ref), which is inputtedthrough the source, to the fourth thin film transistor T₄ through thedrain thereof in response to the gate signal applied through theprevious gate line Gq⁻¹. In other words, the second thin film transistorT₂ has a conduction path between the reference voltage V_(ref) and thegate of the fourth thin film transistor T₄, and the conduction path iscontrolled by the gate signal applied from the previous gate line.

The third thin film transistor T₃ has a source connected to the drain ofthe first thin film transistor T₁, and a gate and a drain commonlyconnected to the drain of the second thin film transistor, the storagecapacitor Cst and the gate of the fourth thin film transistor T₄. Thethird thin film transistor T₃ transmits the data signal transferredthrough the first thin film transistor T₁ to the fourth thin filmtransistor T₄ as a gate control signal of the fourth thin filmtransistor T₄. In other words, the third thin film transistor T₃ has aconduction path to transfer the data signal from the data line Dp to thegate of the driving transistor, i.e., the fourth thin film transistorT₄. The conduction path of the third thin film transistor T₃ iscontrolled by a state of the second thin film transistor T₂. In otherwords, the third thin film transistor T₃ is turned on when the secondthin film transistor T₂ is turned off. The second and third thin filmtransistors are parallel to each other with respect to the fourth thinfilm transistor, and provide the reference voltage and the data signal,respectively, to the gate of the fourth thin film transistor.

The fourth thin film transistor T₄ has a source connected to the powersupply line for supplying the bias voltage Vdd. The gate of the fourththin film transistor T₄ is connected in common to one terminal of thestorage capacitor Cst, the drain of the second thin film transistor, andthe drain of the third thin film transistor T₃. The fourth thin filmtransistor T₄ has a conduction path to transfer the bias voltage Vdd tothe fifth thin film transistor T₅, and the conduction path is controlledby a gate signal, either the reference signal from the second thin filmtransistor T₂ or the data signal from the third thin film transistor T₃.The fourth thin film transistor T₄ has characteristics similar orsubstantially identical to that of the third thin film transistor T₃.

The fifth thin film transistor T₅ has a source connected to a drain ofthe fourth thin film transistor T₄, a drain connected to the OLED diode,and a gate connected to the previous gate line Gq⁻¹. The fifth thin filmtransistor T₅ outputs the driving signal (e.g., the bias voltage Vdd)provided from the fourth thin film transistor T₄ to the OLED diode inresponse to a gate signal applied through the previous gate line Gq−₁.In other words, the fifth thin film transistor T₅ has a conduction pathcontrolled by the previous gate signal to provide the driving signal tothe OLED diode. The OLED diode emits light in response to the drivingsignal. The OLED diode has two terminals, one connected to the drain ofthe fifth thin film transistor and the other connected to the commonelectrode voltage V_(COM).

The storage capacitor Cst has two terminals, one commonly connected tothe drains of the second and third transistors and the gates of thethird and fourth transistors T₃, T₄ and the other connected to the biasvoltage Vdd. The storage capacitor Cst is charged with the bias voltageVdd and supplies a high-level signal (e.g., the bias voltage V_(dd)) tothe second through fourth thin film transistors T₂-T₄ for one frame.When the previous gate line Gq−₁ has a logic-high signal, the secondtransistor T₂ is turned on so that the voltage stored in the storagecapacitor Cst is discharged through the second transistor of whichsource electrode a reference signal Vref of logic-low is applied to. Asa result, the reference signal Vref is applied to the gate of the fourthtransistor T₄.

Hereinafter, a further detail description of the operation of theswitching and driving transistors of the pixel driving unit in FIG. 3follows. FIGS. 5A and 5B are schematic diagrams for describing theoperation of the pixel driving unit of an OLED device in FIG. 3. Theoperation status of the thin film transistors T₁-T₅ of the pixel drivingunit is shown in FIG. 5A when an active gate signal is applied to theprevious gate line, and also in FIG. 5B when an active gate signal isapplied to the present gate line.

Referring to FIG. 5A, when an active gate signal (e.g., a high levelpulse signal) is applied to the previous gate line, the first, third,fourth and fifth thin film transistors T₁, T₃, T₄, T₅ are turned off andthe second thin film transistor T₂ is turned on. As a result, thereference voltage V_(ref) supplied from the reference voltage line isapplied to the gate of the fourth thin film transistor T₄. The referencevoltage V_(ref) may be defined as following Equation 1.

[V _(gate-off(T1)) ]≦V _(ref) ≦[V _(data,min) +V _(th(T3))]  Equation 1

Here, V_(gate-off(T1)) is a gate-off voltage of the first thin filmtransistor T₁, V_(data,min) is the minimum voltage value of a datasignal applied to the data line D_(p), and V_(th(T3)) is a thresholdvoltage of the third thin film transistor T₃, which is a negativevoltage. In this condition, the gate-off voltage of the fourth thin filmtransistor T₄ becomes the reference voltage Vref, and the gate-offvoltage of the third thin film transistor T₃ also becomes the referencevoltage Vref.

Referring to FIG. 5B, when an active gate signal (e.g., a high levelpulse signal) is applied to the present gate line, the first thin filmtransistor T₁ is turned on and the second thin film transistor T₂ isturned off. As a result, a data signal applied from the data line istransferred to the third thin film transistor T₃ through the conductionpath of the first thin film transistor. The storage capacitor Cstcharged with the data signal voltage Vdata provides a high-level voltageto the gate of the third thin film transistor T₃. Then, the third thinfilm transistor T₃ is turned on and the data signal provided from thefirst thin film transistor T₃ is transferred to the gate of the fourththin film transistor T₄. At this time, an effective gate-source voltageVgs′_((T4)), which determines the intensity of current flowing theconduction path of the fourth thin film transistor T₄, is defined asfollowing Equation 2.

Vgs′ _((T4)) =Vgs _((T4)) +Vth _((T4))  Equation 2

Here, the gate-source voltage Vgs_((T4)) of the fourth thin filmtransistor T₄ is the difference between a gate voltage of the fourththin film transistor T₄ and the bias voltage Vdd, as expressed infollowing Equation 3.

Vgs _((T4)) =Vg _((T4)) −Vdd  Equation 3

Here, the gate voltage Vg_((T4)) of the fourth thin film transistor T₄is the difference between a data voltage (i.e., a voltage value of thedata signal) and a threshold voltage Vth of the third thin filmtransistor T₃, as expressed in following Equation 4.

Vg _((T4)) =Vdata+Vth _((T3))  Equation 4

Here, the threshold voltage Vth_((T3)) of the third thin film transistorT₃ is a negative voltage. Assuming that the characteristics of the thirdand fourth thin film transistors T₃, T₄ are substantially identical toeach other, the threshold voltage Vth of the third thin film transistorT₃ is substantially identical to that of the fourth thin film transistorT₄.

Vth_((T3))=Vth_((T4))  Equation 5

From Equations 2 to 5, the effective gate-source voltage Vgs′_((T4))determining the intensity of current flowing the fourth thin filmtransistor T₄ is obtained as follows:

Vgs′ _((T4)) =Vdata−Vdd  Equation 6

As shown in Equation 6, the effective gate-source voltage Vgs′_((T4)) ofthe fourth thin film transistor T₄ is the difference between the datavoltage Vdata provided through the data line Dp and the bias voltage Vddprovided through the external power supply line. Accordingly, theeffective gate-source voltage Vgs′_((T4)) of the fourth thin filmtransistor T₄ in every pixel of the OLED device is only dependent onintensities of the data voltage Vdata applied through the data line Dpand the bias voltage Vdd applied through the external power supply line.In other words, the effective gate-source voltage Vgs′_((T4)) of thedriving transistor, which determines the intensity of current flowingthe driving transistor, is independent of the threshold voltage Vth ofthe driving transistor (i.e., the fourth thin film transistor T₄).

Therefore, by employing in each pixel driving unit of the OLED devicethe third and fourth thin film transistors T₃, T₄ having thesubstantially same characteristics, the pixel driving unit compensatesfor the characteristics (especially, the threshold voltage Vth) of thefourth thin film transistor T₄, which would be different in differentpixels. As a result, the driving transistor of each pixel driving unitprovides the OLED diode with current having the substantially sameintensity in response to a same data signal even if the drivingtransistor (e.g., the fourth thin film transistor T₄) of a pixel has adifferent threshold voltage from that of another pixel.

Referring to FIG. 6, there is provided a pixel driving unit for an OLEDdevice according to a second exemplary embodiment of the presentinvention. In this embodiment, the pixel driving unit does not require aseparate line for providing the reference voltage Vref. In the circuitdiagram of FIG. 6, the pixel driving unit for an OLED device includesfirst to third thin film transistors T₁-T₃ each having a switchingfunction, fourth and fifth thin film transistors T₄, T₅ each having adriving function, a storage capacitor Cst, and an OLED diode connectedto a common electrode voltage V_(COM). Each pixel driving unit isaligned in a region surrounded by gate lines for transferring gatesignals and data lines for transferring data signals. In FIG. 6, theparts equivalent to those in FIG. 3 are represented with like referencenumerals and description thereof is omitted to avoid duplication.

In this embodiment, a gate signal applied through the current gate lineG_(q) serves as the reference voltage Vref for the second thin filmtransistor T₂. For example, the drain of the second thin film transistorT₂ is connected to the current gate line G_(q) in common with the gateof the first thin film transistor T₁.

In the operation of the pixel driving unit, when an active gate signal(e.g., a high-level pulse signal) is applied to the previous gate lineGq−₁, the first, third, fourth and fifth thin film transistors T₁, T₃,T₄, T₅ are turned off and the second thin film transistor T₂ is turnedon. As a result, the reference voltage Vref is applied to the gate ofthe fourth thin film transistor T₄ through the second thin filmtransistor T₂. Since the reference voltage Vref is a gate signal appliedto the present gate line that is inactive when the previous gate line isselected to receive an active gate signal, the reference voltage Vref isan off-level signal.

When an active gate signal is applied to a present gate line G_(q), thefirst thin film transistor T₁ is turned on, so that a data signal (e.g.,a high-level voltage signal) applied from the data line D_(p) to thesource of the first thin film transistor T₁ is transferred to the thirdthin film transistor T₃ through the conduction path of the first thinfilm transistor T₁. At this time, the storage capacitor Cst charged withthe data signal voltage Vdata provides a high-level voltage to the gateof the third thin film transistor T₃ to turn on the third thin filmtransistor T₃. Then, the data signal transmitted through the first andthird thin film transistors T₁, T₃ is supplied to the gate of the fourththin film transistor T₄. Therefore, as mentioned above, no separatereference line is necessary in this embodiment.

FIG. 7 is a circuit diagram illustrating a pixel driving unit for anOLED device according to a third exemplary embodiment of the presentinvention. Referring to FIG. 7, the pixel driving unit includes first tothird thin film transistors T₁ to T₃ each having a switching function, afourth thin film transistors T₄ having a driving function, a storagecapacitor Cst, and an OLED diode connected to a common electrode voltageV_(COM), which constitute a unit pixel of the OLED device. The pixeldriving unit is aligned in a region surrounded by adjacent gate linesfor transferring gate signals and adjacent data lines for transferringdata signals. In FIG. 7, the parts equivalent to those in FIG. 3 arerepresented with like reference numerals and description thereof isomitted to avoid duplication. In the pixel driving unit of thisembodiment, the fifth thin film transistor T₅ is omitted from the pixeldriving unit according to the first embodiment of the present inventionin FIG. 3.

When an active gate signal is applied to the previous gate line Gq⁻¹,the first, third and fourth thin film transistors T₁, T₃, T₄ are turnedoff and the second thin film transistor T₂ is turned on. As a result, areference voltage Vref supplied from a reference voltage line to thesource of the second thin film transistor T₂ is applied to the gate ofthe fourth thin film transistor T₄. In this embodiment, the referencevoltage Vref is the same as the one described in Equation 1.

Also, when an active gate signal is applied to a present gate lineG_(q), the first thin film transistor T₁ is turned on so that a datasignal applied through the data line D_(p) to the source of the firstthin film transistor T₁ is transferred to the third thin film transistorT₃ through the conduction path of the first thin film transistor T₁. Atthis time, since the storage capacitor Cst charged with the data signalvoltage Vdata provides a high-level voltage to the gate of the thirdthin film transistor T₃, the third thin film transistor T₃ is turned on.Thus, the data signal passing through the first and third thin filmtransistors T₁, T₃ is supplied to the gate of the fourth thin filmtransistor T₄.

In like manner, the pixel driving unit of this embodiment compensatesits pixel driving function for any variance in the threshold voltage Vthof the fourth thin film transistor T₄, which would have a characteristicdifferent from that of a fourth thin film transistor in another pixeldriving unit of the same OLED device. With such compensation, the pixeldriving unit provides the OLED diode with the same current as a drivingsignal in response to the same data signal independent of the varyingcharacteristics of the driving transistor in different pixel drivingunits.

FIG. 8 is a circuit diagram illustrating multiple pixel driving units ofan OLED device arranged in association with gate and data linesaccording to an embodiment of the present invention. In this embodiment,the multiple pixel driving units each have the structure identical tothat of the embodiment in FIG. 3. Referring to FIG. 8, the OLED devicehas multiple pixel driving units arranged in a matrix form respectivelycorresponding to the pixels of the OLED device. The OLED device in thisembodiment has “n” pixel driving units (i.e., “n” pixels) in a columnand “n” gate lines G₁-G_(n) each associated with a corresponding one ofthe “n” pixel driving units. The OLED device sequentially provides agate signal, as a scanning signal, to the respective gate lines.

In addition to the “n” gate lines G₁, G₂ . . . G_(n−1), and G_(n) forthe “n” pixel driving units, the OLED device also has a dummy gate lineG₀ to supply a gate signal to the gates of the second and fifth thinfilm transistors T₂ and T₅ of the pixel driving unit in association withthe first gate line G₁. The dummy gate line G₀ is synchronized with then_(th) gate line G_(n). By synchronizing the dummy gate line G₀ with then_(th) gate line G_(n), the dummy gate line G₀ is prevented fromremaining in a floating state.

Alternatively, the dummy gate line G_(o) may receive a separate gatesignal from a gate driver instead of being synchronized with the n_(th)gate line G_(n). In other words, a gate driver sequentially provides the“n” gate signals to the “n” gate lines, respectively, to drive aselected gate line. When the gate driver provides a gate signal to then_(th) gate line G_(n), it also provides the same gate signal to thedummy gate line G_(o) simultaneously. Thus, the dummy gate line G_(o)has the same effect as being synchronized with the n_(th) gate lineG_(n) and is prevented from remaining in the floating state.

In the above embodiments of FIGS. 3 to 8, the pixel driving units (orthe pixels) of an OLED device are arranged in a matrix form such thateach pixel driving unit is defined by the adjacent gate lines expandedin the row direction and arranged parallel with each other in the columndirection and the adjacent data lines expanded in the column directionand arranged parallel with each other in the row direction. Also, apower supply line is expanded in the column direction and arrangedparallel with the data lines to provide a bias voltage to a drivingtransistor of the respective pixel driving units.

In this configuration, such arrangement of the power supply lineparallel with the corresponding data line may cause the “cross-talk”phenomenon in the OLED device. In other words, when the power supplyline is expanded in the column direction parallel with the data line asshown in FIG. 8, a full-level bias voltage is applied to a first pixeldriving unit, but the level of the bias voltage may be gradually loweredas it is applied to lower pixels. As a result, a voltage difference mayexist between the gate-source voltage V_(gs1(T4)) of the fourth thinfilm transistor T₄ in the first pixel driving unit and the gate-sourcevoltage V_(gsn(T4)) of the fourth thin film transistor T₄ in the n_(th)pixel driving unit. Due to the difference of the gate-source voltagebetween the pixel driving units, a voltage difference may exist betweensources of the fourth thin film transistors in the respective pixeldriving units, even if data voltage having the same level is applied tothe pixel driving units. Such “cross-talk” phenomenon may be increasedin the lower pixels, thereby causing deterioration in display quality ofthe OLED device.

FIG. 9 is a circuit diagram illustrating a pixel driving unit for anOLED device according to a fourth exemplary embodiment of the presentinvention. In this embodiment, the power supply line Vdd is expanded inthe row direction and arranged parallel with the gate lines, therebyeffectively reducing the “cross-talk” phenomenon. Referring to FIG. 9,the pixel driving unit according to the fourth embodiment of the presentinvention includes first to third thin film transistors T₁-T₃ eachhaving a switching function, fourth and fifth thin film transistors T₄,T₅ each having a driving function, a storage capacitor Cst, and an OLEDdiode connected to a common electrode voltage V_(COM), which are theelements mainly constituting a unit pixel of the OLED device. The pixeldriving units are arranged in a matrix form in the OLED device such thateach pixel driving unit is defined in a region surrounded by twoadjacent gate lines each for transferring a gate signal and two adjacentdata lines each for transferring a data signal. In FIG. 9, the partsequivalent to those in FIG. 3 are represented with like referencenumerals and description thereof is omitted to avoid duplication. Inthis embodiment, the gate of the fifth thin film transistor T₅ isconnected to the present gate line G_(q), so that the fifth thin filmtransistor T₅ is turned on or off in response to a gate signal providedthrough the present gate signal.

In the operation of the pixel driving unit in FIG. 9, when an activegate signal (e.g., a high-level pulse signal) is applied to the previousgate line Gq⁻¹, the first, third and fourth thin film transistors T₁,T₃, T₄ are turned off and the second and fifth thin film transistors T₂,T₅ are turned on. As a result, the reference voltage Vref supplied froma reference voltage line is applied to the gate of the fourth thin filmtransistor T₄. The reference voltage Vref used in this embodiment is thesame as the one described in Equation 1.

When an active gate signal is applied to the present gate line G_(q),the first thin film transistor T₁ is turned on so that a data signalapplied from the data line D_(p) to the source of the first thin filmtransistor T₁ is transferred to the drain of the third thin filmtransistor T₃. At this time, the storage capacitor Cst charged with thedata signal voltage Vdata provides a high-level voltage to the gate ofthe third thin film transistor T₃, so that the third thin filmtransistor T₃ is turned on. Thus, the data signal passing through thefirst and third thin film transistors T₁, T₃ is supplied to the gate ofthe fourth thin film transistor T₄. Then, the fourth thin filmtransistor T₄ provides the bias voltage to the fifth thin filmtransistor through its conduction path in response to the data signalapplied to its gate. The fifth thin film transistor T₅ is turned offwhen a signal of the present gate line G_(q) becomes logic high.Subsequently, when the previous gate line signal becomes logic high andthe present gate line signal becomes logic low, the fifth thin filmtransistor T₅ is turned on and maintains the active (i.e., turned-on)state until the next frame.

In this embodiment, as described above, the “cross-talk” phenomenon iseffectively eliminated by disposing the power supply line expanded inthe row direction parallel with the gate lines, while compensating thedriving operation of the pixel driving units for varying characteristicsof the fourth thin film transistor T₄ in different pixel driving units.

FIG. 10 is a circuit diagram illustrating a pixel driving unit for anOLED device according to a fifth exemplary embodiment of the presentinvention. In FIG. 10, the same parts as those shown in FIG. 3 arerepresented with light reference numerals. In this embodiment, the powersupply line is expanded in the row direction parallel with the gatelines and an additional thin film transistor T₆ is provided between thefifth thin film transistor and the OELD diode.

Referring to FIG. 10, the pixel driving unit includes first to thirdthin film transistors T₁-T₃ each having a switching function, fourth tosixth thin film transistors T₄-T₆ each having a driving function, astorage capacitor Cst, and the OLED diode connected to a commonelectrode voltage V_(COM), which are the elements mainly constituting aunit pixel of the OLED device. Each of the pixel driving units of theOLED device is disposed in a region surrounded by adjacent gate lineseach for transferring a gate signal and adjacent data lines each fortransferring a data signal. In this embodiment, the fifth thin filmtransistor T₅ is a P-type thin film transistor and has the gateconnected to the previous gate line G_(q−1) so as to be turned on or offin response to a gate signal provided through the previous gate signal.Also, the sixth thin film transistor T₆ is an N-type thin filmtransistor and has the source connected to the drain of the fifth thinfilm transistor T₅, the drain connected to the OLED diode, and the gateconnected to the present gate line G_(q). The sixth thin film transistorT₆ is thus turned on or off in response to a gate signal applied throughthe present gate signal G_(q).

In operation, when an active gate signal is applied to the previous gateline Gq−1, the first, third, fourth, fifth and sixth thin filmtransistors T₁, T₃, T₄, T₅, T₆ are turned off and the second thin filmtransistor T₂ is turned on. Thus, the reference voltage Vref suppliedfrom a reference voltage line is applied to the gate of the fourth thinfilm transistor T₄. The reference voltage Vref is the same as the onedescribed in Equation 1.

When an active gate signal is applied to the present gate line G_(q),the first thin film transistor T₁ is turned on so that a data signalapplied from the data line D_(p) to the source of the first thin film T₁transistor is transferred to the drain of the third thin film transistorT₃ through the conduction path of the first thin film transistor T₁.Also, the second thin film transistor is turned off because the previousgate line is inactivated. Thus, the data signal passing through thefirst and third thin film transistors T₁, T₃ is supplied to the gate ofthe fourth thin film transistor T₄. Accordingly, the conduction path ofthe fourth thin film transistor is controlled by the data signalprovided from the source of the third thin film transistors T₃.

In this embodiment, as mentioned for the fourth embodiment above, the“cross-talk” phenomenon existing between adjacent pixels arranged in thecolumn direction is effectively eliminated by aligning the power supplyline expanded in the row direction parallel with the gate lines.

In the pixel driving units of the first through fifth embodiments of thepresent invention, the first and second thin film transistors T₁, T₂ areN-type thin film transistors and the third to fifth thin filmtransistors T₃, T₄, T₅ are P-type thin film transistors. However, thepresent invention is not limited to such configuration. For example, apixel driving unit of the present invention may include first to fourththin film transistors T₁-T₄ which one is P-type thin film transistorsand a fifth thin film transistor T₅ which is an N-type thin filmtransistor.

FIG. 11 is a circuit diagram illustrating a pixel driving unit for anOLED device according to a sixth exemplary embodiment of the presentinvention, and FIG. 12 is a graphical view of signal waveforms appliedto the pixel driving unit in FIG. 11. Referring to FIG. 11, the pixeldriving unit includes first to third thin film transistors T₂₁-T₂₃ eachhaving a switching function, fourth and fifth thin film transistors T₂₄,T₂₅ each having a driving function, a storage capacitor Cst, and an OLEDdiode connected to a common electrode voltage V_(COM), which constitutea unit pixel of the OLED device. The pixel driving unit is disposed in aregion surrounded by adjacent gate lines for transferring gate signalsand adjacent data lines for transferring data signals. Since the firstto fourth thin film transistors T₂₁-T₂₄ are P-type thin filmtransistors, they are each turned on when a gate signal having a levellower than its threshold voltage is applied to the gate of thecorresponding one of the first to fourth thin film transistors T₂₁-T₂₄.The fifth thin film transistor T₂₅ is an N-type thin film transistor,thus it is turned on when a gate signal having a level higher than itsthreshold voltage is applied to the gate of the fifth thin filmtransistor T₂₅.

As shown in FIG. 12, the gate signals respectively applied to the gatelines are inversed signals. In other words, since the first thin filmtransistor T₂₁ is a P-type thin film transistor, the first thin filmtransistor T₂₁ is maintained inactive when the gate signal applied tothe current gate line G_(q) has a high-level. In contrast, the firstthin film transistor T₂₁ is maintained active when the gate signalapplied to the current gate line G_(q) has a low-level (i.e., active lowsignal). In order to supply such inversed gate signals to the pixeldriving unit for the OLED device, an inverter is provided in a gatedriver that sequentially outputs the gate signals.

In the operation of the pixel driving unit, when the previous gate lineis activated (i.e., a low-level gate signal is applied to the previousgate line Gq−1 and a high-level gate signal is applied to the currentgate line), the first, third, fourth and fifth thin film transistors T₁,T₃, T₄, T₅ are turned off and the second thin film transistor T₂ isturned on. Thus, the reference voltage Vref is applied to the gate ofthe fourth thin film transistor T₄. In this case, the reference voltageVref may be defined as following Equations 7 and 8.

V_(ref)<V_(gate-off(T21))  Equation 7

Here, V_(gate-off(T21)) is a gate-off voltage of the first thin filmtransistor T₂₁.

V _(ref) <[V _(data,min) +V _(th(T23))]  Equation 8

Here, V_(data,min) is the minimum voltage value of a data signal appliedto the data line Dp, and V_(th(T23)) is the threshold voltage of thethird thin film transistor T₂₃.

Next, when the current gate line G_(q) is activated (i.e., a low-levelgate signal is applied to the current gate line G_(q) and a high-levelgate signal is applied to the previous gate line G_(q−1)), the firstthin film transistor T₂₁ is turned on so that a data signal applied tothe source of the first thin film transistor T₂₁ is transferred to thedrain of the third thin film transistor T₂₃. At this time, since thestorage capacitor Cst charge with the data signal voltage provides ahigh-level voltage to the gate of the third thin film transistor T₂₃,the third thin film transistor T₂₃ is turned on. Thus, the data signalpassing through the first and third thin film transistor T₂₁, T₂₃ issupplied to the gate of the fourth thin film transistor T₂₄. At thistime, an effective gate-source voltage Vgs′_((T24)) determining theintensity of current flowing the fourth thin film transistor T₂₄ isrepresented by following Equation 9.

Vgs′ _((T24)) =Vgs _((T24)) −Vth _((T24))  Equation 9

Here, the gate-source voltage of the fourth thin film transistor T₂₄ isthe difference between the gate voltage of the fourth thin filmtransistor T₂₄ and the bias voltage V_(dd). The gate-source voltage isrepresented by following Equation 10.

Vgs _((T24)) =Vg _((T24)) −Vdd  Equation 10

Here, the gate voltage of the fourth thin film transistor T₂₄ is thedifference between the data voltage and the threshold voltage of thethird thin film transistor T₂₃, and it is represented by followingEquation 11.

Vg _((T24)) =Vdata+Vth _((T23))  Equation 11

Here, since the characteristics of the third and fourth thin filmtransistors T₃, T₄ are substantially identical to each other, thresholdvoltage Vth of the third thin film transistor T₂₃ is the same as that ofthe fourth thin film transistor T₂₄ as follows.

Vth_((T23))=Vth_((T24))  Equation 12

Therefore, from Equations 9 through 12, the effective gate-sourcevoltage of the fourth thin film transistor T₂₄ is obtained as followingEquation 13.

Vgs′ _((T24)) =Vdata−Vdd  Equation 13

As expressed in Equation 13, the effective gate-source voltageVgs′_((T24)) determining the intensity of current flowing the fourththin film transistor T₂₄ is the voltage difference between the datavoltage Vdata applied through the data line Dp and the bias voltageV_(dd) applied through the external power supply line. Accordingly, theeffective gate-source voltage Vgs′_((T24)) of each of the fourth thinfilm transistors T₂₄ respectively disposed in all the pixel drivingunits is only dependent on the intensity of the data voltage Vdataapplied through the data line Dp and the bias voltage V_(dd) appliedthrough the external power supply line. The gate-source voltageVgs′_((T24)) is, however, independent of the threshold voltage Vth ofthe fourth thin film transistor T₂₄.

In this embodiment, the pixel driving unit compensates the drivingoperation of the fourth thin film transistor T₄ for its varyingcharacteristics such that the threshold voltage Vth of a fourth thinfilm transistor T₄ in a pixel driving unit may be different from that ofanother fourth thin film transistor in another pixel driving unit. Withsuch a compensation, the same current is provided to the OLED diode inresponse to the same data signal independent of the threshold voltage ofthe fourth thin film transistor T₄, even when the driving transistors(i.e., the fourth thin film transistors T₂₄) have different thresholdvoltages in different pixel driving units. It is assumed that the pixeldriving unit of the present invention has the third and fourth thin filmtransistors of which characteristics (e.g., threshold voltages) aresimilar or substantially identical to each other.

The thin film transistors employed in the pixel driving units for anOLED device according to the present invention each have a multi-layeredstructure including a semiconductor layer, an insulation layer, aprotection layer and an electrode layer. The semiconductor layerincludes amorphous silicon or polysilicon. The insulation layer includessilicon nitride (SiN_(X)), silicon oxide (SiO₂), aluminum oxide (AL₂O₂)and tantalum oxide (TaOx). The protection layer includes transparentorganic insulating material or insulating material. The electrode layerincludes conductive metal, for example, Al, Cr and Mo. Each of themultiple layers is fabricated as a thin film by using a depositionapparatus, such as a sputtering device and a chemical vapor depositiondevice. Then, the thin films are subjected to a lithography process toform the elements of the pixel driving units for the OLED device.

Of the multiple layers, the semiconductor layer serves as an electricalconduction channel through which electrons are moved, and the electrodelayer includes a source electrode, a drain electrode and a gateelectrode. The source electrode applies a voltage signal to thesemiconductor layer, and the voltage signal traveling the semiconductorlayer is output through the drain electrode. The gate electrode is ameans for controlling (e.g., switching) the current flow from the sourceelectrode to the drain electrode.

Thus, the thin film transistors with such configuration may be used asswitching devices in an active matrix type OLED device. The thin filmtransistors of the active matrix type OLED device each have asemiconductor layer made of material including cadmium selenide (CdSe),hydrogenous amorphous silicon (a-Si:H), or poly crystalline silicon(poly-Si).

The amorphous silicon can be processed at a low temperature with asimple process, so that it has been used for a large-scale device, forexample, a solar cell. In addition, a semiconductor manufacturingprocess using the amorphous silicon may be carried out in a lowtemperature processing system at the maximum temperature of about 350°C., so that the semiconductor device can be easily fabricated in case ofusing the amorphous silicon. However, electrons in the amorphous siliconmove at a very low speed, thereby deteriorating switchingcharacteristics of the thin film transistors. In addition, it isdifficult to integrate driving circuitry controlling the thin filmtransistors at a high speed with the thin film transistors. In contrast,a thin film transistor having a semiconductor layer includingpolysilicon is adapted for the active matrix type OLED device.

Although the thin film transistor having the semiconductor layerincluding polysilicon requires an additional process, the polysiliconthin film transistor that serves as a switching device provided in theactive matrix type OLED device has a response speed faster than that ofthe amorphous silicon thin film transistor. In addition, the polysiliconthin film transistor has superior field effect mobility as compared withthat of the amorphous silicon thin film transistor. The field effectmobility determines a switching speed of the thin film transistor. Theswitching speed of the polysilicon thin film transistor is remarkablyfaster than the switching speed of the amorphous silicon thin filmtransistor.

This is because the polysilicon consists of various grains and has a lowdefect as compared with amorphous silicon. Thus, polysilicon can be usedfor a switching device in a next-generation OLED device having a largescreen while allowing the drive circuitry to be integrated with the thinfilm transistor. The polysilicon thin film transistor may be fabricatedthrough a solid phase crystallization (SPC) process in which amorphoussilicon is crystallized at a high temperature, a metal inducedcrystallization (MIC) process in which heat is applied to metaldeposited on amorphous silicon, or an excimer laser annealing processusing a laser. The excimer laser annealing process can be carried out ata low temperature with using an inexpensive glass substrate, so it cansave manufacturing cost. In addition, the thin film transistormanufactured through the excimer laser annealing process has highmobility of signals, so that the operational characteristic of asemiconductor device is improved.

Hereinafter, methods of fabricating a polysilicon thin film transistorby crystallizing an amorphous silicon thin film transistor using a laserwill be described with reference to the accompanying drawings.

FIGS. 13A and 13B are schematic diagrams each illustrating two thin filmtransistors fabricated according to an exemplary embodiment of thepresent invention. The two thin film transistors are the third andfourth thin film transistors T₃ (or T₂₃), T₄ (or T₂₄) in the pixeldriving unit of the present invention (referring to FIGS. 3-11). Asdescribed above, the third and fourth thin film transistors in the pixeldriving unit have the characteristics similar or substantially identicalto each other. According to a method of fabricating the two thin filmtransistors, a polysilicon thin film transistor is formed bycrystallizing an amorphous silicon thin film transistor using a laserscan, and the third and fourth thin film transistors T₃ and T₄ areformed in the same plane.

Referring to FIG. 13A, the third and fourth amorphous silicon thin filmtransistors T₃ and T₄ are formed on the same plane of a glass substrate.The gate electrodes G of the third and fourth amorphous silicon-thinfilm transistors T₃ and T₄ are formed in parallel to each other, and thesource and drain electrodes S, D of the third and fourth amorphoussilicon-thin film transistors T₃ and T₄ are formed to be collinearlyarranged (i.e., aligned in a same line) in a direction substantiallyperpendicular to the laser scan direction that is substantially parallelto the gate electrodes G. Then, the amorphous silicon thin filmtransistors T₃, T₄ are subjected to the laser scan to be crystallizedand transformed into the polysilicon thin film transistors.

FIG. 13B is a schematic diagram for describing a method of fabricatingthe third and fourth thin film transistors T₃ and T₄ according toanother embodiment of the present invention. The third and fourthamorphous silicon thin film transistors T₃ and T₄ are formed on the sameplane of the glass substrate. Then, the gate electrodes G of the thirdand fourth amorphous silicon thin film transistors T₃ and T₄ arecollinearly formed, and the source and drain electrodes S, D of thethird and fourth amorphous silicon-thin film transistors T₃, T₄ areformed substantially parallel to the laser scan direction. The gateelectrodes G are formed in a collinear direction substantiallyperpendicular to the laser scan direction. Upon being subjected to thelaser scan, the amorphous silicon-thin film transistors are crystallizedand transformed into the polysilicon thin film transistors.

When crystallizing the amorphous silicon-thin film transistors throughthe laser annealing process, a mask for forming a laser beam pattern onthe glass substrate and a zoom lens for exposing a pattern of the maskto the glass substrate by zooming-out the pattern are prepared. Indetail, a laser beam is adjusted to have a uniform distribution, and thelaser beam pattern to be formed on the glass substrate through the maskis determined. For example, a light source is employed to generate aline beam from its tip. In this case, a line-shaped mask is also used.In other method, another mask in addition to the line-shaped mask may beused between the glass substrate and the light source to form a specificpattern.

Then, the laser beam is adjusted to have a selected beam width by meansof a zoom-out lens. In case of employing the line-shaped mask alone, thebeam width is in a range from several millimeters to several centimetersand the length is from several centimeters to several tens centimeters(or up to about 2 meters). In case of employing another mask in additionto the line-shaped mask, the beam width is in a range from severalmicrometers to several millimeters. The amorphous silicon thin filmtransistors are crystallized by radiating the laser beam onto the glasssubstrate while moving the glass substrate or the laser beam on the X-Yplane, so that the amorphous silicon thin film transistors aretransformed into the polysilicon thin film transistors.

FIG. 14 is a plan view of the pixel driving unit for an OLED device inFIG. 3. FIGS. 15 and 16 are cross-sectional views of the pixel drivingunit taken along lines A-A′ and B-B′, respectively, in FIG. 14. As shownin FIGS. 15 and 16, the pixel driving unit is formed in multiple layers,such as an insulation substrate 10 including glass, quartz, and/orsapphire, a blocking layer 20, a gate insulating layer 30, an interlayerdielectric 40, and a passivation layer 50.

Referring to FIGS. 14 to 16, the blocking layer 20 is formed on theglass substrate 20 by depositing silicon oxide on the glass substrate 20at a thickness of about 2000 Å through a plasma-enhanced chemical vapordeposition process. On the blocking layer 20, five thin film transistorsT₁-T₅, one storage capacitor C, and five wirings Gn−1, Gn, DL, Vdd,V_(ref) are formed. The blocking layer 20 is provided to prevent thermalloss while crystallizing an amorphous-silicon layer to form apolysilicon layer.

One pixel driving unit is defined by first and second gate lines Gn−1,Gn extending in a first direction, a data line DL extending in a seconddirection substantially perpendicular to the first direction, and apower supply line Vdd extending in the second direction. A referencevoltage line V_(ref) extends in the first direction and is disposedbetween the first and second gate lines Gn−1, Gn.

In detail, the first gate line Gn−1 turns on/off the first thin filmtransistor T₁ of a pixel driving unit in the previous column, so that aninitial data signal and a gray-scale data signal are provided throughthe data line DL. Also, the first gate line Gn−1 turns on/off the secondand fifth thin film transistors T₂, T₅ of the pixel driving unit in thecurrent column.

The second gate line Gn turns on/off the first thin film transistor T₁of the pixel driving unit in the current column to perform a switchingfunction, so that an initial data signal and a gray-scale data signalare provided to the pixel driving unit through the data line DL. Also,the second gate line Gn turns on/off the second and fifth thin filmtransistors T₂, T₅ of a pixel driving unit in the next column. Throughthe power supply line Vdd, the maximum value of a display signal isconstantly applied in the form of direct current to the pixel drivingunit. Through the reference voltage line, the reference voltage Vref isapplied to the second thin film transistor T₂.

Referring to FIG. 14, the first thin film transistor T₁ includes a firstactive pattern 110 disposed at an area adjacent to a cross point betweenthe second gate line Gn and the data line DL, a gate electrode 112extending from the second gate line Gn and passing above the firstactive pattern 110, a source electrode 114 extending from the data lineDL and making contact with the first active pattern 110 aligned at oneside of the gate electrode 112, and a first drain electrode 116 makingcontact with the first active pattern 110 aligned at the other side ofthe gate electrode 112. The gate electrode 112 and the source electrode114 of the first thin film transistor T₁ are connected to the secondgate line Gn and the data line DL, respectively.

The second thin film transistor T₂ includes a second active pattern 120,a gate electrode 122 extending from the first gate line Gn−1 and passingabove the second active pattern 120, a source electrode 124 extendingfrom the reference voltage line V_(ref) and making contact with thesecond active pattern 120 aligned at one side of the gate electrode 122,and a drain electrode 126 making contact with the second active pattern120 aligned at the other side of the gate electrode 122.

The third thin film transistor T₃ includes the first active pattern 110,a gate electrode 132 extending from a metal line G_(M), which is formedwhen the first gate line Gn−1 is formed, and passing above the firstactive pattern 110, a source electrode 134 extending from the referencevoltage line Vref and making contact with the first active pattern 110aligned at one side of the gate electrode 132, and a drain electrode 136making contact with the first active pattern 110 aligned at the otherside of the gate electrode 132.

The fourth thin film transistor T₄ includes a third active pattern 140,a gate electrode 142 extending from the metal line G_(M) and passingabove the third active pattern 140, a source electrode 144 extendingfrom the reference voltage line V_(ref) and making contact with thethird active pattern 140 aligned at one side of the gate electrode 142,and a drain electrode 146 making contact with the third active pattern140 aligned at the other side of the gate electrode 142.

The fifth thin film transistor T₅ includes a fourth active pattern 150,a gate electrode 152 extending from the first gate line Gn−1 and passingabove the fourth active pattern 150, a source electrode 154 extendingfrom the drain electrode 147 of the fourth thin film transistor T₄ andmaking contact with the fourth active pattern 150 aligned at one side ofthe gate electrode 152, and a drain electrode 156 making contact withthe fourth active pattern 150 aligned at the other side of the gateelectrode 152 and making contact with an anode electrode of the OLEDdevice.

In this embodiment, the first and second thin film transistors T₁, T₂are N-type thin film transistors, and the third to fifth thin filmtransistors T₃-T₅ are P-type thin film transistors. The storagecapacitor Cst is defined by the metal line G_(M) formed when forming thefirst gate line and the power supply line Vdd formed above the metalline G_(M). The storage capacitor Cst constantly maintains data voltagefor one frame time.

After forming the five thin film transistors T₁-T₅, the storagecapacitor Cst, and the five wirings Gn−1, Gn, DL, Vdd, V_(ref), an ITOanode electrode is formed on the top portion of the pixel driving unitand exposed through an opening. Also, a hole transfer layer, a lightemitting layer and an electron transfer layer are sequentially formed onan organic insulating wall of the pixel driving unit, and a cathodeelectrode is formed thereon. In this embodiment, the third and fourththin film transistors are disposed in a direction parallel to the dataline, so that the characteristics of the third and fourth thin filmtransistors become similar or substantially identical to each other whenthe amorphous silicon-thin film transistors are crystallized through alaser scan process.

As describe above, the pixel driving unit according to the presentinvention compensates for the threshold voltage of a driving thin filmtransistor in the respective pixel driving units of an OLED device. Theeffective gate-source voltage of the driving thin film transistor isonly dependent on a data voltage and a bias voltage externally appliedand independent of the threshold voltage of the driving thin filmtransistor, so that each OLED diode receives a driving signal with thesame intensity in response to the same data signal.

While the invention has been described with reference to the exemplaryembodiments, it will be understood by those skilled in the art thatvarious changes may be made and equivalents may be substituted forelements thereof without departing form the scope of the invention. Inaddition, many modifications may be made to adapt a particular situationor material to the teachings of the invention without departing from theessential scope thereof. Therefore, it is intended that the inventionmay not be limited to the particular embodiment disclosed as the bestmode contemplated for carrying out this invention, but that theinvention will include all embodiments falling within the scope of theintended claims.

1-35. (canceled)
 34. The organic light emitting display device of claim26, wherein the driving, first and second switching transistors are thinfilm transistors each having a source and a drain forming a conductionpath and a gate receiving a gate signal to control the conduction path.35. A method for fabricating a semiconductor device for providing apixel driving signal in an organic light emitting display device,comprising: providing an insulation substrate; forming on the insulationsubstrate a first amorphous silicon thin film transistor for providingthe pixel driving signal to an organic light emitting diode; forming onthe insulation substrate a second amorphous silicon thin film transistorfor transferring a data signal to control a switching function of thefirst amorphous silicon thin film transistor; crystallizing the firstand second amorphous silicon thin film transistors by performing a laserscan on the first and second amorphous silicon thin film transistor; andtransforming the first and second amorphous silicon thin film transistorinto first and second polysilicon thin film transistors, respectively,by consummating the crystallizing step, wherein the first and secondpolysilicon thin film transistors have characteristics substantiallyidentical to each other.
 36. The method of claim 35, wherein the formingthe first amorphous silicon thin film transistor includes forming a gateelectrode of the first amorphous silicon thin film transistor in adirection substantially parallel to a laser scan direction, and theforming the second amorphous silicon thin film transistor includesforming a gate electrode of the second amorphous silicon thin filmtransistor in the direction substantially parallel to the laser scandirection.
 37. The method of claim 36, wherein the forming the firstamorphous silicon thin film transistor includes forming source and drainelectrodes of the first amorphous silicon thin film transistor in acollinear direction substantially perpendicular to the laser scandirection, and the second amorphous silicon thin film transistorincludes forming source and drain electrodes of the second amorphoussilicon thin film transistor in the collinear direction substantiallyperpendicular to the laser scan direction.
 38. The method of claim 35,wherein the forming the first amorphous silicon thin film transistorincludes forming a gate electrode of the first amorphous silicon thinfilm transistor in a collinear direction substantially perpendicular toa laser scan direction, and the forming the second amorphous siliconthin film transistor includes forming a gate electrode of the secondamorphous silicon thin film transistor in the collinear directionsubstantially perpendicular to the laser scan direction.
 39. The methodof claim 38, wherein the forming the first amorphous silicon thin filmtransistor includes forming source and drain electrodes of the firstamorphous silicon thin film transistor in a direction substantiallyparallel to the laser scan direction, and the second amorphous siliconthin film transistor includes forming source and drain electrodes of thesecond amorphous silicon thin film transistor in the directionsubstantially parallel to the laser scan direction.
 40. The method ofclaim 35, further including forming a mask that forms a laser beampattern on the insulation substrate.
 41. The method of claim 40, furtherincluding exposing a pattern of the mask to the insulation substrate byzooming-out the pattern.